The present invention relates in general to the field of telephone networks and communications, and more particularly to echo cancellation in telecommunications systems.
The presence of reflected voice signals or xe2x80x9cechoxe2x80x9d in telephone lines is a well-known phenomenon. Modern telephone systems employ echo cancellers at various points in a transmission system to eliminate such undesirable reflected voice signals. An early example of an echo canceller is described in U.S. Pat. No. 3,500,000, issued Mar. 10, 1970.
Hybrid circuits are a common source of impedance mismatch that gives rise to the signal reflection problem that may be heard as an echo of the speaker""s own voice. In addition to hybrid circuits, telephone systems have other inherent sources of reflection and signal feedback that can give rise to undesirable echo transmissions. For example, speaker phones and xe2x80x9chands-freexe2x80x9d mobile phones can acoustically couple or xe2x80x9cfeedbackxe2x80x9d a portion of the sound from the phone""s loudspeaker into its microphone. Conventional echo cancellers can eliminate undesirable echoes from any such additional sources, when the echo signals are correlated, as well as from the ordinary hybrid circuit.
To facilitate an understanding of the echo phenomenon, reference is made to FIG. 1 showing a simplified transmission system of the prior art, which is designated generally by reference numeral 10. The system 10 is shown connecting telephone A to telephone B through a network N. Phone A is connected by line 11 to a hybrid circuit HA which in turn is connected to an echo canceller EA by line 12. The echo canceller EA is connected to the network N by line 13. Similarly, phone B is connected through hybrid circuit HB and echo canceller EB to the network N via lines 14, 15 and 16. The lines 11 and 14 typically consist of conventional two-wire subscriber loops (or xe2x80x9clocal loopsxe2x80x9d) through which analog voice signals are conducted in both directions. The hybrid circuits HA and HB separate the two-way voice signals on lines 11 and 14 to provide separate transmit and receive signals on the respective pairs of the four-wire lines 12 and 15. A hybrid circuit can be part of the subscriber""s equipment or part of the phone company""s equipment.
Whether an echo is perceptible, and therefore objectionable, depends upon the delay from original transmission to receipt of the reflected signal. In the example of FIG. 1, if a party using phone A is speaking, the signal must travel the distance from phone A to hybrid circuit HB on the opposite side of the network N and be reflected back to phone A. To prevent the return of such echo signal to phone A, echo canceller EB superimposes an inverted copy of the echo signal on the line 16 to cancel the actual echo signal reflected by hybrid circuit HB. The echo canceller EB senses the duration for transmission from it to hybrid HB and reflection back to precisely time the cancellation function. Thus, the party speaking into phone A will not hear any annoying echoes. Similarly, echo canceller EA may be employed to remove the echo of speech transmitted by phone B and caused by signal reflection at hybrid circuit HA.
More recently, digital transmission has become commonplace in telecommunications networks. As a result, sophisticated digital echo cancellers have been developed to subtract out echoes caused by reflections at various points in the transmission system. Such digital echo cancellers are well known in the art, an illustrative example being described in U.S. Pat. No. 5,418,849.
In addition to transmitting digitized voice signals, telephone systems are being used increasingly for digital data transmission, as when computers communicate with each other. A telephone technology known as Integrated Services Digital Network (ISDN) provides uniform standards and protocols for computers to send and receive digital data through the twisted-pair copper wires of the conventional local loop at relatively high transmission rates compared to xe2x80x9cmodemxe2x80x9d technology. An important application for ISDN technology is to provide a relatively high-speed connection to the Internet via the two-wire local loop of a conventional telephone. Unlike digital voice transmissions, ISDN data transmissions do not require echo cancellation. Conventional digital echo cancellers must be disabled so that they can pass ISDN data and other digital data transmissions without applying echo cancellation.
One end of a digital transmission system is depicted in the simplified block diagram of FIG. 2 and designated generally by reference numeral 20. A phone A used by a xe2x80x9cnear-end talkerxe2x80x9d is connected to a HYBRID circuit by a conventional subscriber loop 21 for sending and receiving analog voice signals. The HYBRID circuit provides separate communication paths 22 and 23 for xe2x80x9csendxe2x80x9d and xe2x80x9creceivexe2x80x9d signals, respectively. A conventional device known as a CODEC (coder-decoder) converts analog signals on send line 22 to digital signals on send line 24, and converts digital signals on receive line 25 to analog signals on receive line 23. A digital echo canceller 26 communicates with the telephone network (not shown) via send line 27 and receive line 28. Pulse-code-modulated (PCM) signals are communicated on lines 24, 25, 27 and 28 in accordance with network standards. The network interconnects the near-end talker using phone A with a far-end talker (not shown).
Echo canceller 26 is employed to eliminate the echo of the far-end talker""s voice reflected on send path 22 by the HYBRID circuit. The far-end talker""s voice signal is received on line 28 by the echo canceller 26, sensed internally and passed through as an output on line 25. From the signal on line 28 the echo canceller 26 estimates an echo signal expected to be returned on line 24. The echo canceller 26 then subtracts the estimated echo signal from the actual echo signal. The resulting signal, which may include some xe2x80x9cresidualxe2x80x9d echo, is further processed internally by the echo canceller 26 to produce an essentially echo-free output on line 27.
In the United States, a digital multiplexing system is employed in which a first level of multiplexed transmission, known as T1, combines 24 digitized voice channels over a four-wire cable (one pair of wires for xe2x80x9csendxe2x80x9d signals and one pair for xe2x80x9creceivexe2x80x9d signals). The conventional echo canceller 26 of FIG. 2 is shown operating on a single PCM voice transmission line prior to multiplexing (or xe2x80x9cmuxingxe2x80x9d) for network transmission. The digital coding produced by the CODEC on line 24 provides 8,000 samples per second of the analog signal on line 22, each sample being represented by an 8-bit binary number. Thus, the transmission rate on line 24 is 64,000 bits per second (64 kbps).
The conventional bit format on the T1 carrier is known as DS1 (i.e., first level multiplexed digital service or digital signal format), which consists of consecutive frames, each frame having 24 PCM voice channels (or DS0 channels) of 8 bits each. Each frame has an additional framing bit for control purposes, for a total of 193 bits per frame. The T1 transmission rate is 8,000 frames per second or 1.544 megabits per second (Mbps). The frames are assembled for T1 transmission using a technique known as time division multiplexing (TDM), in which each DS0 channel is assigned one of 24 sequential time slots within a frame, each time slot containing an 8-bit word.
Transmission through the network of local, regional and long distance service providers involves sophisticated call processing through various switches and a hierarchy of multiplexed carriers. At the pinnacle of conventional high-speed transmission is the synchronous optical network (SONET), which uses fiber-optic media and is capable of transmission rates in the gigabit range (in excess of one billion bits per second). After passing through the network, the higher level multiplexed carriers are demultiplexed (xe2x80x9cdemuxedxe2x80x9d) back down to individual DS0 lines, decoded and connected to individual subscriber phones.
Echo cancelling is commonly applied at the DS0 level. It has been conventional practice to provide 24 echo cancellers per T1 line so that each DS0 channel has a dedicated echo canceller. However, as digital data transmission over telephone lines has increased (e.g., for ISDN data traffic), the percentage of DS0 channels needing echo cancellation has decreased. Unlike digitized voice, such digital data communication in a DS0 channel does not require echo cancellation. When digital data is detected, typically the call processing system has had to route the call to special trunk groups not equipped with echo cancellers, or when echo cancellation is equipped on a dedicated basis, has had to disable the echo canceller on that particular DS0 channel.
Echo cancellation may be applied at various points within a transmission system. It is common to apply echo cancellation on the network side (rather than subscriber side or xe2x80x9caccess sidexe2x80x9d) of a conventional voice circuit switch operating on T1 lines. By way of illustration, FIG. 3 shows such a switch in a block diagram. The switch is designated generally by reference numeral 30 and includes an access-side port device 31, a switch core 32 and a network-side port device 33. Such switches are common in the public telephone network and facilitate the basic routing and interconnection of ordinary telephone calls and data communications over telephone lines. Multiplexers 34 and 35 are provided on the network side of the switch 30 to mux up the signals to higher rates for transmission through conventional high-speed media. For example, DS3 transmission is typically carried by a coaxial cable and combines 28 DS1 signals at 44.736 Mbps. An OC3 optical fiber carrier, which is at a low level in the optical hierarchy, combines three DS3 signals at 155.52 Mbps, providing a capacity for 2016 individual voice channels in a single fiber-optic cable. SONET transmissions carried by optical fiber are capable of even higher transmission rates.
The switch 30 is simplified in FIG. 3 to show it operating on a single DS1 line 36, but it will understood that switching among many such lines actually occurs so that calls on thousands of individual subscriber lines can be routed through the switch on their way to their ultimate destinations. Port device 31 demultiplexes the signals on DS1 line 36 to provide 24 corresponding DS0 appearances to ports of the switch core 32. The switch core 32 includes a complex matrix of electronic switches and control circuits that route the individual DS0 lines on the access side to other DS0 lines on the network side. The signals emerging on the network side of the switch core 32 are muxed back up to the DS1 level and transmitted further on line 37. DS1 carrier line 37 and other such lines (not shown) are muxed up to the DS3 level by multiplexer 34 for transmission on line 38. Similarly, DS3 carrier line 38 and other such lines (not shown) are muxed up to an optical transmission level, such as OC3, by multiplexer 35 for transmission by SONET carrier 39.
FIG. 4 schematically depicts an arrangement of echo cancellers as commonly employed in prior art systems. A voice circuit switch 40 is shown in block diagram form but will be understood to include a switch core and port devices like those of the switch 30 of FIG. 3. The arrangement of FIG. 4 also shows two of a plurality of multiplexers 41 and 42, each muxing up 28 DS1 transmissions to the DS3 level. A group of 28 echo canceller cards, designated collectively by reference numeral 43, services the lines entering multiplexer 41, and a group of 28 echo canceller cards, designated collectively by reference numeral 44, services the lines entering multiplexer 42. The switch 40 has a plurality of T1 lines 45 entering from the access side. A corresponding number of T1 lines 46 emerge from the switch 40 arranged in groups of 28 to correspond to respective multiplexers.
Each of the echo canceller cards of the groups 43 and 44 contains 24 echo cancellers since each card services one T1 transmission line carrying 24 voice channels. Typically, the circuitry of each echo canceller is implemented in a single integrated circuit chip. Thus, it will be appreciated that each T1 line 46 has a dedicated echo canceller card, and each DS0 channel has a dedicated echo canceller chip. As an alternative to the arrangement of FIG. 4, some prior art systems have voice circuit switches with internal echo cancellers dedicated on a DS0 basis. However, whether the echo cancellers are of the internal or external type, the prior art systems typically provide a dedicated echo canceller for each DS0 channel. In some cases, groups of T1 lines are not equipped with echo cancellers and are used exclusively for digital data transmissions.
In many instances individual echo cancellers in prior art systems are maintained in a disabled state and merely pass through the DS0 signal transmissions without applying echo cancelling. This occurs either because the transmission delay is sufficiently short that echo cancellation is not needed or because digital data is being carried by the DS0 line. Also, echo cancellation may have been applied at a different point in the transmission network and thus is not needed at this particular point in the system. Because each echo canceller is dedicated to a particular DS0 line, a significant percentage of echo cancelling equipment remains quiescent at all times.
In accordance with the present invention, an echo canceller system for use in a digital telephone transmission system is provided in an efficient equipment architecture. Echo cancellers are pooled and selectively interconnected by call processing control through a pool switch matrix to individual transmission lines only in the event that a determination is made that the line requires echo cancellation.
The echo canceller pooling arrangement of the present invention permits efficient use of echo cancellers on an as needed basis. A relatively small number of echo cancelers can effectively service a relatively large number of individual transmission lines.
The pool switch matrix optionally can be configured to dynamically route either access-side transmissions or network-side transmissions to echo canceller inputs to cancel echoes coming from either direction.
The invention optionally can provide additional system efficiencies, such as combining multiplexer stages in a port device on one side of a voice circuit switch to enable direct connection of a fiber-optic cable to the multiplexed output of the port device.